Numerous cosmetic compositions for the skin in the form of an emulsion exhibit a pH of greater than 6, mainly for reasons of formulation stability. This is because the stability of the emulsions with regard to coalescence is often reinforced using ionic surfactants, which have to be formulated at a pH at least equal to 6 in order to be in the ionized form.
For example, the document EP 0 641 557 describes oil-in-water emulsions formed of oily globules of defined size provided with a lamellar liquid crystal coating and comprising a lipophilic surface-active agent and a hydrophilic surface-active agent. These emulsions comprise a fatty acid in order to improve their stability and this acid, due to its low pKa, has to be formulated at a pH at least equal to 6 in order to be in the ionized form. The document EP 0 705 593 describes oil-in-water emulsions formed of oily globules of defined size provided with a lamellar liquid crystal coating and comprising a lipophilic surface-active agent, a hydrophilic surface-active agent and an ionic amphiphilic compound conferring on the emulsion a pH ranging from 5.5 to 7.5. These amphiphilic compounds do not make it possible to obtain an emulsion at a low pH.
Furthermore, many gelling agents which provide for the stability of the emulsions with regard to creaming or sedimentation are found to be greatly restricted in their gelling properties at a pH of less than 6.
However, attempts are being made to formulate emulsions having a pH of less than 6, in particular for reasons of harmlessness as such a pH is closer to the pH of the skin, itself between 4.5 and 6, and the microbiological protection of formulations exhibiting this pH range is possible with lower levels of preservatives.
In addition, the compositions exhibiting this pH range make it possible to more easily formulate water-soluble active principles in the acid form, such as salicylic acid or hydroxy acids.
It thus appears necessary to have available cosmetic emulsions, particularly cosmetic and dermatologic emulsions, which are stable at a pH ranging from 3 to 5.5 and which exhibit a broad range of textures extending from fluid to cream.